Can I discriminate against smokers when hiring child care workers?

I am not familiar with Minnesota law, but I am unaware of any statute which contains a general prohibition against discrimination in employment, or which would require you to treat smokers equally.  While many jurisdictions have their own local laws which may expand protections, the Federal laws against discrimination in hiring generally apply only to employers of a certain size, and protect workers from discrimination on the basis of things like gender, race, national origin, etc.  If you, for example, didn't want to hire anybody who was a Red Sox fan, I know of no reason why you couldn't follow that practice.